Electric motors that utilize permanent magnets and magnet coils are known, having been disclosed in JP2001-298982A and JP2003-111483A, for example.
In the prior art electric motors, motor control is carried out using an on/off signal from a digital magnetic sensor. Specifically, the timing for reversing the polarity of the voltage applied to the magnet coil is determined using the on/off signal from the digital magnetic sensor.
Magnetic sensors having analog output (so-called analog magnetic sensors) are also available. However, where an analog magnetic sensor is used to control a motor, considerable error in sensor output can occur due to manufacturing errors of various kinds among motors, making it impossible in some instances to carry out motor control satisfactorily. Examples of manufacturing errors among motors that can have an effect on output of an analog magnetic sensor are error in the installation location of the magnetic sensor; error in the location of the N pole/S pole boundary due to magnetization error of the permanent magnet; and error in mounting location of elements inside the magnetic sensor. However, to date there have yet to be devised satisfactory technology for achieving accurate motor control using analog magnetic sensors, while taking such errors into consideration. This problem is not limited to cases where analog magnetic sensors are used; the problem is encountered also in cases where digital magnetic sensors having multi-value analog output are used.
Incidentally, drive systems employing multiple electric motors have been implemented in robots, mobile objects (e.g. vehicles), and the like.
However, in the past it was necessary to provide a large number of control lines between the individual electric motors and the overall system controller for the purpose of controlling the multiple electric motors.